1. Field of the Invention
The present invention relates to a lift method and apparatus for moving an arm structure such as a boom of a crane.
2. Description of the Background Art
Various devices for moving an arm structure such as a lifting arm currently exist. These devices typically employ connecting plates joining relatively movable first and second arm structures. Known arrangements of this type require an excessive amount of force from a moving device that is attached to one of the arm structures and the connecting plate. Furthermore, the conventional art provides a limited range of movement of the second arm structure relative to the first arm.
For example, the conventional art of FIG. 9 shows a first arm structure 10 and a second arm structure 12 fastened together by connecting plate 14. Arm 12 pivots about pin 18, mounted in plate 14. A hydraulic piston and cylinder arrangement 16 is connected to the second arm structure 12 and to connecting plate 14. As shown with the dashed lines in FIG. 9, when the second arm structure 12 approaches a fully extended position, the movement of the second arm structure 12 with respect to the first arm structure 10 is limited by the orientation of the hydraulic piston and cylinder arrangement 16 relative to arm structure 12.
In the fully extended position, the hydraulic piston and cylinder arrangement 16 contacts the pin connection 18 that fastens the second arm structure 12 to the connecting plate 14. This limits the range of motion of arm 12.
As illustrated in FIG. 9, the conventional arm structure arrangement is highly inefficient when the second arm structure 12 approaches a fully extended position. This inefficiency can be explained with reference to the angle xcex8 between longitudinal axis B1 of moving device 16 and longitudinal axis A1 of arm structure 12. This angle xcex8 affects the respective forces generated by the second arm structure 12 (denoted as F12) and the hydraulic piston and cylinder arrangement 16 (denoted as F16).
As the arm 12 approaches a fully extended position, a longitudinal axis B1 of the piston and cylinder 16 is nearly parallel to a longitudinal axis A1 of arm 12, where angle xcex8 substantially approaches 0xc2x0. The extending force F16 generated by the hydraulic piston and cylinder arrangement 16 is nearly perpendicular to force F12, which is the component of force tending to rotate arm 12 counterclockwise due to the weight of arm 12 plus any loads carried by arm 12. Force F16, which has component forces F16a and Fl6b, therefore, cannot efficiently counteract force F12.
Accordingly, a need in the art exists for a lift method and apparatus for moving an arm structure that substantially increases a range of motion of a second arm structure relative to a first arm structure and which also provides a moving device with a mechanical advantage whereby work required to move the second arm structure relative to the first arm structure is substantially reduced while lifting capacity of the moving device is substantially increased.
Accordingly, it is a primary object of the present invention to provide a lift method and apparatus for moving an arm structure which substantially increases a range of motion of a second arm structure relative to a first arm structure while providing a mechanical advantage for the moving device whereby work required to move the second arm structure relative to the first arm structure is substantially reduced while lifting capacity of the moving device is substantially increased.
It is a further object of the present invention to provide a lift method and apparatus for moving an arm structure which substantially lowers the overall height and center of gravity of a first arm structure, a second arm structure, and a connecting plate system when the first arm structure and second arm structure are in a stowed position.
A further object of the present invention is to provide a lift method and apparatus for moving an arm structure which includes means for displacing a moving device relative to a said first arm whereby the moving device is provided with a mechanical advantage with respect to forces which oppose the moving device that are created by the arm structure.
A further object of the present invention is to provide a lift method and apparatus for moving an arm structure which substantially increases a range of motion of the second arm structure relative to the first arm structure when at least the second arm structure substantially approaches an extended position.
An additional object of the present invention is to provide a lift method and apparatus for moving an arm structure wherein the apparatus and method can be employed in a crane environment such that a first arm structure is a riser while the second arm structure is a boom connected to the riser by a riser connecting plate. A specific object is to provide a moving device in the form of a hydraulic piston and cylinder arrangement which is provided with a substantially increased lifting capacity due to a displacing device which movably connects the hydraulic piston and cylinder arrangement to the connecting plate.
Another object of the present invention is to provide a lift method and apparatus for moving an arm structure that substantially maximizes the lift of the moving device as the moving device moves through its operating range.
A further object of the present invention is to provide a lift method and apparatus for moving an arm structure that allows for increased lifting capacity, weight savings, manufacturing cost savings, and a reduction in work performed by the moving device or a reduction in hydraulic pressure. Another object of the present invention is to make more compact the mechanical elements which connect the moving device to the arm structure, while substantially achieving a shorter load radius for a given boom length or second arm structure length.
These and other objects of the present invention are fulfilled by providing a lift apparatus for moving an arm structure comprising: a first arm structure; a second arm structure having first longitudinal axis, said second arm structure being rotatable relative to the first arm structure; a moving device for applying a force along a second longitudinal axis; and at least one means for displacing the moving device relative to the first arm and the second arm during at least a portion of the range of motion of the second arm for increasing a range of motion of the second arm structure relative to the first arm structure and for providing said moving device with an increased mechanical advantage for moving the second arm structure relative to the first arm structure.
In addition, these and other objects of the present invention are also accomplished by providing a method of operating an apparatus comprising a first arm structure, a second arm structure pivotally attached to the first arm structure, and a moving device for creating relative pivotal movement between the first and second arm structures, the method comprising: activating the moving device to move the first and second arms relative to each other, and during at least a portion of the range of relative movement of the first and second arms, displacing the moving device with respect to at least one of the first arm or the second arm so as to improve the mechanical advantage of the moving device in generating the relative movement.